Sympathetic Nerve Terminals Research Articles

Facilitation of transmitter release from rat sympathetic neurons via presynaptic P2Y1 receptors

P2Y1 receptor antagonists are being developed as antithrombotics. In the vasculature, P2Y1 receptors are not only expressed in platelets, but also in the endothelium where they indirectly mediate vasorelaxation. By blockade of these latter effects, P2Y1 antagonists may cause untoward effects. Here, we investigated whether vascular sympathetic axon terminals also possess P2Y1 receptors by determining [3H]noradrenaline release from superior cervical ganglion neurons in cell culture. ADP inhibited electrically evoked release, but this effect was reverted into facilitation when either P2Y12 receptors were blocked by cangrelor or their signalling cascade was interrupted by pertussis toxin. The facilitation by ADP was also observed with K+ stimulation in the presence of tetrodotoxin, was mimicked by 2-methlythio-ATP, and was prevented by suramin, reactive blue 2, and MRS2179. The facilitation by ADP was abolished by the phospholipase C inhibitor {type:entrez-nucleotide,attrs:{text:U73122,term_id:4098075,term_text:U73122}}U73122, but was not affected by cholera toxin to down-regulate Gs proteins, by H-7 to block protein kinases, or by thapsigargin to deplete intracellular Ca2+ stores. In patch clamp recordings, activation of P2Y1 receptors led to an inhibition of KCNQ channels, but the KCNQ channel modulators retigabine and XE 991 did not alter electrically evoked noradrenaline release or its facilitation by ADP. These results show that presynaptic P2Y1 receptors mediate facilitation of transmitter release from sympathetic nerve terminals via phospholipase C, and interference with this mechanism by P2Y1 antagonists can be expected to counteract the previously described vasoconstrictive action of these drugs.

Tripartite function of ATP in vascular signalling

Tripartite function of ATP in vascular signalling

An in vitro assay for the assessment of the effects of an organophosphate, paraoxon, and a triazine, atrazine, on the heart of the gilthead sea bream ( Sparus aurata)

The effects of paraoxon and atrazine on the spontaneously beating auricle, isolated from the heart of Sparus aurata, were assessed. Paraoxon, 5 μM, eliminated the atria contraction within 28.4 ± 2.8 min, an effect which was fully reversed by 15 μM atropine, an antagonist of muscarinic cholinergic receptors. The IC 50 was estimated to be 3.2 ± 1.5 μΜ. Atrazine, 50 and 100 μM, induced a 22.5 ± 3.2 and 32.9 ± 2.3% increase in the force of auricle contraction, caused by excitation of sympathetic synaptic terminals releasing adrenaline. This effect was reversed by 50 μM propranolol, a blocker of β-adrenoreceptors. The results have shown that both sympathetic and parasympathetic nerve terminals are activated by atrazine. Also, the auricle contraction is mainly under sympathetic control, while the frequency is dominated by cholinergic system. Finally, the detailed parameters of the auricle contraction estimated during exposure to specific pesticides, force, frequency, time-response curves and electromechanical coupling can be further used to assess and compare the toxic effects of other compounds, anticholinesterases for example, on the heart of the fish.

Modified Sympathetic Nerve System Activity with Overexpression of the Voltage-dependent Calcium Channel β3 Subunit

N-type voltage-dependent calcium channels (VDCCs) play determining roles in calcium entry at sympathetic nerve terminals and trigger the release of the neurotransmitter norepinephrine. The accessory beta3 subunit of these channels preferentially forms N-type channels with a pore-forming CaV2.2 subunit. To examine its role in sympathetic nerve regulation, we established a beta3-overexpressing transgenic (beta3-Tg) mouse line. In these mice, we analyzed cardiovascular functions such as electrocardiography, blood pressure, echocardiography, and isovolumic contraction of the left ventricle with a Langendorff apparatus. Furthermore, we compared the cardiac function with that of beta3-null and CaV2.2 (alpha1B)-null mice. The beta3-Tg mice showed increased expression of the beta3 subunit, resulting in increased amounts of CaV2.2 in supracervical ganglion (SCG) neurons. The beta3-Tg mice had increased heart rate and enhanced sensitivity to N-type channel-specific blockers in electrocardiography, blood pressure, and echocardiography. In contrast, cardiac atria of the beta3-Tg mice revealed normal contractility to isoproterenol. Furthermore, their cardiac myocytes showed normal calcium channel currents, indicating unchanged calcium influx through VDCCs. Langendorff heart perfusion analysis revealed enhanced sensitivity to electric field stimulation in the beta3-Tg mice, whereas beta3-null and Cav2.2-null showed decreased responsiveness. The plasma epinephrine and norepinephrine levels in the beta3-Tg mice were significantly increased in the basal state, indicating enhanced sympathetic tone. Electrophysiological analysis in SCG neurons of beta3-Tg mice revealed increased calcium channel currents, especially N- and L-type currents. These results identify a determining role for the beta3 subunit in the N-type channel population in SCG and a major role in sympathetic nerve regulation.

Contrasting effects of presynaptic α2-adrenergic autoinhibition and pharmacologic augmentation of presynaptic inhibition on sympathetic heart rate control

Presynaptic alpha2-adrenergic receptors are known to exert feedback inhibition on norepinephrine release from the sympathetic nerve terminals. To elucidate the dynamic characteristics of the inhibition, we stimulated the right cardiac sympathetic nerve according to a binary white noise signal while measuring heart rate (HR) in anesthetized rabbits (n = 6). We estimated the transfer function from cardiac sympathetic nerve stimulation to HR and the corresponding step response of HR, with and without the blockade of presynaptic inhibition by yohimbine (1 mg/kg followed by 0.1 mg.kg(-1).h(-1) iv). We also examined the effect of the alpha2-adrenergic receptor agonist clonidine (0.3 and 1.5 mg.kg(-1).h(-1) iv) in different rabbits (n = 5). Yohimbine increased the maximum step response (from 7.2 +/- 0.8 to 12.2 +/- 1.7 beats/min, means +/- SE, P < 0.05) without significantly affecting the initial slope (0.93 +/- 0.23 vs. 0.94 +/- 0.22 beats.min(-1).s(-1)). Higher dose but not lower dose clonidine significantly decreased the maximum step response (from 6.3 +/- 0.8 to 6.8 +/- 1.0 and 2.8 +/- 0.5 beats/min, P < 0.05) and also reduced the initial slope (from 0.56 +/- 0.07 to 0.51 +/- 0.04 and 0.22 +/- 0.06 beats.min(-1).s(-1), P < 0.05). Our findings indicate that presynaptic alpha2-adrenergic autoinhibition limits the maximum response without significantly compromising the rapidity of effector response. In contrast, pharmacologic augmentation of the presynaptic inhibition not only attenuates the maximum response but also results in a sluggish effector response.

Sympathetic histamine exerts different pre‐ and post‐synaptic functions according to the frequencies of nerve stimulation in guinea pig vas deferens

Histamine (HA) was found to be present in the sympathetic nerve terminals of guinea pig hearts and vasa deferentia in our previous study; however, little is known about the functions of this neurogenic HA. In this study, we used guinea pig vasa deferentia to investigate the pre- and post-synaptic functions of HA evoked by different frequencies of sympathetic nerve stimulation. We found that sympathetic nerve stimulation could evoke HA release, which was independent to mast cell degranulator compound 48/80 and mast cell stabilizer cromolyn, but was highly sensitive to Na(+) channel blocker tetrodotoxin and chemical sympathectomy with 6-hydroxydopamine. The neurogenically released HA evoked by 12.5 Hz of nerve stimulation activated only pre-synaptic H(3) receptors and mediated pre-synaptic inhibitory effects, while under 25 or 50 Hz stimulation condition, HA simultaneously activated both pre-synaptic H(3) receptors and post-synaptic H(1) receptors. However, the direct contractile responses evoked by sympathetic HA via H(1) receptors were observed at 50 Hz. HA release and HA-mediated contractile responses upon sympathetic nerve stimulation were significantly inhibited by pre-treatment of histidine decarboxylase inhibitor alpha-fluoromethylhistidine. Furthermore, application of exogenous HA could mimic these pre- and post-synaptic effects. Our findings indicate that HA in sympathetic neurons acts as a neurotransmitter and its functions vary from pre-synaptic inhibition, to post-synaptic facilitation, to direct post-synaptic contractile responses according to sympathetic nerve stimulation frequencies.

The Future of Cardiovascular Imaging in the Diagnosis and Management of Heart Failure, Part 1

Radiographic, ultrasound, nuclear, and magnetic resonance methods have become indispensable in the management of heart failure (HF). Imaging is widely used in decision making in HF, not only in relation to left ventricular (LV) systolic and diastolic function but also in the selection of medical, device, and surgical therapy in HF and valvular heart disease. Future developments in the care of advanced heart disease, including stem cell therapy, device therapy to control remodeling, and percutaneous valve interventions, as well as the need to identify subclinical heart disease, are likely to expand this use. Moreover, the epidemic of diabesity (diabetes and obesity) will augment the existing epidemic of HF just when it appeared to have peaked.1 Large numbers of patients will need information from imaging to guide clinical decision making (Table 1). The sources of this information will need to be expeditious, inexpensive, and preferably objective and quantitative.2 View this table: Table 1. Established HF: What the Clinician Needs Although some of the measurements in HF patients are structural (LV mass and geometry), the main components of an imaging assessment in HF will continue to be primarily functional (LV ejection fraction, size, filling pressures, filling characteristics, and right ventricular [RV] function). A number of new technologies will enhance the future accuracy and reliability of these measures. ### LV Volumes and Ejection Fraction Routine techniques (contrast ventriculography, 2D echocardiography) provide real-time imaging in standard imaging planes. The only traditional method that has escaped this limitation has been radionuclide ventriculography, in which the ejection fraction is calculated from scintigraphic counts, but this introduces the separate potential problems of attenuation and overlying chambers. #### Limitations of 2D Imaging Two-dimensional imaging approaches require expert acquisition and observers and have limited reliability in obtaining appropriate cut planes in sequential studies. The limitation of requiring imaging planes in the correct axis is particularly an issue for 2D echocardiography, which …

Abnormal Sympathetic Innervation of Viable Myocardium and the Substrate of Ventricular Tachycardia After Myocardial Infarction

The aim of this study was to characterize the relationship between impaired sympathetic innervation and arrhythmia with noninvasive biologic imaging in an animal model of post-infarct ventricular tachycardia (VT). Innervation might be abnormal in the normally perfused borderzone of myocardial infarction, contributing to myocardial catecholamine overexposure and arrhythmogenic risk. Myocardial infarction was induced by mid-left anterior descending coronary artery balloon occlusion in 11 pigs. Positron emission tomography (PET) of tissue perfusion and catecholamine uptake and storage was performed with [13N]-ammonia and [11C]-epinephrine 4 to 12 weeks later. Magnetic resonance imaging and invasive electrophysiology (electroanatomic mapping, basket catheter, VT inducibility) were performed within 1 week of PET. When compared with a normal database of 9 healthy animals, reduced perfusion was observed in 37 +/- 7% of the left ventricle (LV). Epinephrine retention was reduced in 44 +/- 7% of LV, resulting in a perfusion/innervation mismatch of 7 +/- 4% LV. Sustained monomorphic VT was inducible in 7 of 11 animals. These animals showed a larger perfusion/innervation mismatch (10 +/- 4% vs. 4 +/- 2% LV for animals without VT; p = 0.02). Regionally, the degree of perfusion/innervation mismatch did not correlate with wall thickness or thickening but showed a significant correlation with reduced myocardial voltage (r = 0.93; p = 0.001) and with the site of earliest VT activation (chi-square 13.1; p < 0.001). Noninvasive mapping of cardiac sympathetic nerve terminals reveals regionally impaired catecholamine uptake and storage in the normally perfused borderzone after experimental myocardial infarction. These areas might be useful to characterize the individual risk for ventricular arrhythmia.

Endocannabinoids and the Regulation of Bone Metabolism

In mammals, including humans, bone metabolism is manifested as an ongoing modelling/remodelling process whereby the bone mineralised matrix is being continuously renewed. Recently, the main components of the endocannabinoid system have been reported in the skeleton. Osteoblasts, the bone forming cells, and other cells of the osteoblastic lineage, as well as osteoclasts, the bone resorbing cells, and their precursors, synthesise the endocannabinoids anandamide and 2-arachidonoylglycerol (2-AG). CB(1) cannabinoid receptors are present in sympathetic nerve terminals in close proximity to osteoblasts. Activation of these CB(1) receptors by elevated bone 2-AG levels communicates brain-to-bone signals as exemplified by traumatic brain injury-induced stimulation of bone formation. In this process, the retrograde CB(1) signalling inhibits noradrenaline release and alleviates the tonic sympathetic restrain of bone formation. CB(2) receptors are expressed by osteoblasts and osteoclasts. Their activation stimulates bone formation and suppresses bone resorption. CB(2)-deficient mice display a markedly accelerated age-related bone loss. Ovariectomy-induced bone loss can be both prevented and rescued by a CB(2) specific agonist. Hence, synthetic CB(2) ligands, which are stable and orally available, provide a basis for developing novel anti-osteoporotic therapies, free of psychotropic effects. The CNR2 gene (encoding CB(2)) in women is associated with low bone mineral density, offering an assay for identifying females at risk of developing osteoporosis.

Impaired activation of celiac ganglion neurons in vivo after damage to their sympathetic nerve terminals

Because damage to sympathetic nerve terminals occurs in a variety of diseases, we tested the hypothesis that nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo. First, we measured the effect of nerve terminal damage produced by the sympathetic nerve terminal toxin 6-hydroxydopamine (6-OHDA) on ganglionic levels of several neurotrophins thought to promote neurotransmission. 6-OHDA-induced nerve terminal damage did not decrease the expression of neurotrophin-4 or brain-derived neurotrophic factor mRNA in the celiac ganglia but did decrease the ganglionic content of both nerve growth factor protein (nadir = -63%) and the mRNA of the alpha-3 subunit of the nicotinic cholinergic receptor (nadir = -49%), a subunit required for neurotransmission. Next, we tested whether this degree of receptor deficiency was sufficient to impair activation of celiac ganglia neurons. Impaired fos mRNA responses to nicotine administration in the celiac ganglia of 6-OHDA-pretreated rats correlated temporally with suppressed expression of functional nicotinic receptors. We verified by Fos protein immunohistochemistry that this ganglionic impairment was specific to principal ganglionic neurons. Last, we tested whether centrally initiated ganglionic neurotransmission is also impaired following nerve terminal damage. The principal neurons in rat celiac ganglia were reflexively activated by 2-deoxy-glucose-induced glucopenia, and the Fos response in the celiac ganglia was markedly inhibited by pretreatment with 6-OHDA. We conclude that sympathetic nerve terminal damage per se is sufficient to impair ganglionic neurotransmission in vivo and that decreased nicotinic receptor production is a likely mediator.

L-dihydroxyphenylserine (Droxidopa) in the treatment of orthostatic hypotension

Neurogenic orthostatic hypotension is a cardinal feature of generalised autonomic failure and commonly is the presenting sign in patients with primary autonomic failure. Orthostatic hypotension can result in considerable morbidity and even mortality and is a major management problem in disorders such as pure autonomic failure, multiple system atrophy and also in Parkinson's disease. Treatment is ideally two pronged, using non-pharmacological and pharmacological measures. Drug treatment ideally is aimed at restoring adequate amounts of the neurotransmitter noradrenaline. This often is not achievable because of damage to sympathetic nerve terminals, to autonomic ganglia or to central autonomic networks. An alternative is the use of sympathomimetics (that mimic the effects of noradrenaline, but are not identical to noradrenaline), in addition to other agents that target physiological mechanisms that contribute to blood pressure control.L-threo-dihydroxyphenyslerine (Droxidopa) is a pro-drug which has a structure similar to noradrenaline, but with a carboxyl group. It has no pressor effects in this form. It can be administered orally, unlike noradrenaline, and after absorption is converted by the enzyme dopa decarboxylase into noradrenaline thus increasing levels of the neurotransmitter which is identical to endogenous noradrenaline. Experience in Caucasians and in Europe is limited mainly to patients with dopamine beta hydroxylase deficiency. This review focuses on two studies performed in Europe, and provides information on its efficacy, tolerability and safety in patients with pure autonomic failure, multiple system atrophy and Parkinson's disease. It also addresses the issue of whether addition of dopa decarboxylase inhibitors, when combined with l-dopa in the treatment of the motor deficit in Parkinson's disease, impairs the pressor efficacy of Droxidopa.

Centrally administered N-methyl- d-aspartate evokes the adrenal secretion of noradrenaline and adrenaline by brain thromboxane A 2-mediated mechanisms in rats

Plasma adrenaline mainly originated from adrenaline-containing cells in the adrenal medulla, while plasma noradrenaline reflects the release from sympathetic nerves in addition to the secretion from noradrenaline-containing cells in the adrenal medulla. The present study was undertaken to characterize the source of plasma catecholamines induced by centrally administered N-methyl- d-aspartate with regard to the brain prostanoid, using urethane-anesthetized rats. Intracerebroventricularly (i.c.v.) administered N-methyl- d-aspartate (1.0, 5.0, 10.0 nmol/animal) dose-dependently elevated plasma levels of noradrenaline and adrenaline. The N-methyl- d-aspartate (5.0 nmol/animal, i.c.v.)-induced elevation of both catecholamines was reduced by dizocilpine maleate (5 nmol/animal, i.c.v.), a non-competitive N-methyl- d-aspartate receptor antagonist. Indomethacin (0.6 and 1.2 μmol/animal, i.c.v.), an inhibitor of cyclooxygenase, dose-dependently reduced the N-methyl- d-aspartate (5.0 nmol/animal, i.c.v.)-induced elevation of both catecholamines. The N-methyl- d-aspartate-induced response was dose-dependently attenuated by furegrelate (0.9 and 1.8 μmol/animal, i.c.v.), an inhibitor of thromboxane A 2 synthase. Furthermore, the acute bilateral adrenalectomy abolished the N-methyl- d-aspartate-induced responses, indicating that the source of increase in plasma noradrenaline evoked by N-methyl- d-aspartate is due to secretion from the adrenal gland and not due to release from sympathetic nerve terminals. These results suggest that centrally administered N-methyl- d-aspartate induces the secretion of noradrenaline and adrenaline from adrenal medulla by the brain thromboxane A 2-mediated mechanisms in rats.

Cannabinoid receptors and the regulation of bone mass

A functional endocannabinoid system is present in several mammalian organs and tissues. Recently, endocannabinoids and their receptors have been reported in the skeleton. Osteoblasts, the bone forming cells, and osteoclasts, the bone resorbing cells, produce the endocannabinoids anandamide and 2-arachidonoylglycerol and express CB2 cannabinoid receptors. Although CB2 has been implicated in pathological processes in the central nervous system and peripheral tissues, the skeleton appears as the main system physiologically regulated by CB2. CB2-deficient mice show a markedly accelerated age-related bone loss and the CNR2 gene (encoding CB2) in women is associated with low bone mineral density. The activation of CB2 attenuates ovariectomy-induced bone loss in mice by restraining bone resorption and enhancing bone formation. Hence synthetic CB2 ligands, which are stable and orally available, provide a basis for developing novel anti-osteoporotic therapies. Activation of CB1 in sympathetic nerve terminals in bone inhibits norepinephrine release, thus balancing the tonic sympathetic restrain of bone formation. Low levels of CB1 were also reported in osteoclasts. CB1-null mice display a skeletal phenotype that is dependent on the mouse strain, gender and specific mutation of the CB1 encoding gene, CNR1.

Discrepancy between glucose metabolism and sympathetic nerve terminals in a patient with metastatic paraganglioma

Discrepancy between glucose metabolism and sympathetic nerve terminals in a patient with metastatic paraganglioma

Medical Decision Making

Topical cocaine is used to confirm the clinical diagnosis of ocular sympathetic denervation, or Horner Syndrome (HS). Cocaine blocks re-uptake of norepinephrine (NE) by sympathetic nerve terminals in the iris dilator muscle, transiently increasing its concentration in the synaptic junction. Norepinephrine activates alpha1 receptors in the iris dilator to cause pupil dilation. In HS, cocaine fails to dilate the affected pupil as much as the unaffected pupil, but its indirect action makes it a weak dilator, and the test can give equivocal results. Cocaine is also a controlled substance and therefore difficult to obtain. A practical and reliable alternative to cocaine is apraclonidine, an ocular hypotensive agent that has a weak direct action on alpha1 receptors and therefore minimal to no clinical effect on the pupils of normal eyes. Patients with HS have denervation supersensitivity of the alpha1 receptors in the iris stroma of the affected eye, making the pupil dilator responsive to apraclonidine. In patients with HS, reversal of anisocoria occurs after bilateral instillation of apraclonidine 1% or 0.5%. Two cases that demonstrate this effect are reported. Apraclonidine should be considered a candidate to replace cocaine in the pharmacologic diagnosis of HS if a gold-standard comparison study confirms these results.

Regulation of Skeletal Remodeling by the Endocannabinoid System

Since the discovery of the endocannabinoid system, its presence and involvement have been reported in a handful of biological systems. Recently, the skeleton has been identified as a major endocannabinoid target through both the neuronal CB1 and predominantly peripheral CB2 cannabinoid receptors. CB1 is present in sympathetic nerve terminals in bone, whereas CB2 is expressed in osteoblasts and osteoclasts, the respective bone-forming and -resorbing cells. Furthermore, the skeleton appears as the main system physiologically regulated by CB2. CB2-deficient mice show a markedly accelerated age-related bone loss and the CB2 locus in women is associated with low bone density and osteoporotic fractures. Since activation of CB2 attenuates experimentally induced bone loss by inhibiting bone resorption and stimulating bone formation, and because synthetic cannabinoids are stable and orally available, a therapy based on synthetic CB2 agonists is a promising novel target for antiosteoporotic drug development.

EFFECT OF PERIPHERAL SYMPATHETIC NERVE DYSFUNCTION ON SALT SENSITIVITY OF ARTERIAL PRESSURE

1. Dysregulation of peripheral sympathetic pathways contributes to some forms of salt-dependent hypertension. However, at the present time it is not known whether salt-induced activation of sympathetic nerves or loss of normal sympathoinhibitory responses to salt-induced volume expansion contributes to neurogenic salt-dependent hypertension. The present study was performed to the test the hypothesis that loss of peripheral sympathetic nerve function results in salt-dependent hypertension. 2. The effect of three pharmacological interventions of sympathetic nerve function on the long-term salt-sensitivity of mean arterial pressure (MAP) were measured: (i) blockade of ganglionic transmission with hexamethonium (HEX; n = 5); (ii) destruction of sympathetic nerve terminals with guanethidine (GUAN; n = 7); and (iii) alpha-adrenoceptor blockade with two specific antagonists, namely prazosin (PRAZ; n = 7) and terazosin (TERAZ; n = 8). 3. Mean arterial pressure and heart rate were measured 24 h/day by radiotelemetry in conscious rats during 5 days of normal and 7 days of high (HNa) dietary sodium intake. Despite marked increases in both sodium and water intake during 7 days of the HNa diet, no statistically significant changes in MAP were observed in HEX, GUAN, PRAZ or TERAZ groups. 4. We conclude that loss of peripheral sympathetic neural pathways alone does not cause salt-dependent hypertension in the rat.

INFLUENCE OF HYPERTENSION ON DYSFUNCTION OF REGULATION OF ADRENERGIC NEUROTRANSMISSION IN SHR.Cg‐Leprcp/NDmcr RATS

SUMMARY 1 Our recent studies of the tail artery of SHR.Cg-Leprcp/NDmcr (SHR-cp) rats showed dysfunction of presynaptic inhibitory adenosine-receptor on the sympathetic nerve and promotion of adenyl purine release from the endothelium. We examined the influence of hypotensive agents as an initial approach to understanding whether hypertension participates in these two changes in vascular function. 2 In the tail artery of SHR-cp rats treated with amlodipine (8 mg/kg, p.o.) and moxonidine (4 mg/kg, p.o.) for 10 weeks, 2-chloroadenosine, the adenosine receptor agonist, did not inhibit the contractions evoked by electrical nerve stimulation (1 Hz). In this artery, the release of adenyl purines evoked by noradrenaline was significantly decreased. Treatments with amlodipine and moxonidine increased the amount of plasma adenyl purines and decreased the systolic blood pressure. 3 Dysfunction of prejunctional adenosine-receptor on the sympathetic nerve terminal of SHR-cp rats’ tail artery seems not to be directly caused by hypertension. The extracellular level of ATP in SHR-cp rats seems to be regulated by blood pressure in a complicated manner.

EFFECTS OF DIPEPTIDYL PEPTIDASE IV INHIBITION ON ARTERIAL BLOOD PRESSURE

1. The aim of the present study was to determine whether inhibition of dipeptidyl peptidase IV (DPP IV) elevates arterial blood pressure and whether any such effect is dependent on genetic background, the sympathetic nervous system and Y(1) receptors. The rationale behind this study was that: (i) neuropeptide (NP) Y(1-36) and peptide YY(1-36) (PYY(1-36)) are endogenous Y(1) receptor agonists and are metabolised by DPP IV to NPY(3-36) and PYY(3-36), which are not Y(1) but rather selective Y(2) receptor agonists; (ii) Y(1) receptors mediate vasoconstriction, whereas Y(2) receptors have little effect on vascular tone; (iii) vaso-constrictor effect of the Y(1) receptor is enhanced in spontaneously hypertensive rats (SHR) compared with normotensive Wistar-Kyoto (WKY) rats; and (iv) NPY(1-36) is released from sympathetic nerve terminals. 2. We examined the effects of acute administration of 3-N-[(2S,3S)-2-amino-3-methylpentanoyl]-1,3-thiazolidine (P32/98; a DPP IV inhibitor) on arterial blood pressure in anaesthetized adult SHR and WKY rats in the absence and presence of either captopril, hydralazine or chlorisondamine to lower basal mean arterial blood pressure (MABP) by different mechanisms (inhibition of angiotensin-converting enzyme, direct vasodilation and ganglionic blockade, respectively). 3. In naïve SHR with severely elevated basal blood pressures (MABP = 176 +/- 3 mmHg; n = 4), i.v. boluses (1, 3 and 10 mg/kg) of P32/98 did not affect blood pressure. 4. When basal blood pressure was reduced by pretreatment of SHR with either captopril (30 mg/kg, i.v.; MABP = 116 +/- 3 mmHg; n = 9) or hydralazine (5 mg/kg, i.p.; MABP = 84 +/- 3 mmHg; n = 7), P32/98 (1, 3 and 10 mg/kg) caused significant dose-related increases in arterial blood pressure (4 +/- 2, 10 +/- 2 and 12 +/- 3 mmHg in the captopril-pretreated group, respectively (P < 0.01); 5 +/- 2, 8 +/- 3 and 11 +/- 4 mmHg in the hydralazine-pretreated group, respectively (P < 0.01)). 5. The increases in arterial blood pressure induced by P32/98 in captopril- or hydralazine-pretreated SHR were entirely blocked by pretreatment with the selective Y(1) receptor antagonist N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-d-arginine amide (BIBP 3226; 6 mg/kg per h). 6. When basal blood pressure was reduced in SHR by pretreatment with chlorisondamine (10 mg/kg, s.c.; MABP = 108 +/- 4 mmHg; n = 7), inhibition of DPP IV with P32/98 did not affect arterial blood pressure. Basal heart rate in chlorisondamine-treated SHR was significantly reduced compared with naïve SHR, captopril-pretreated SHR and hydralazine-pretreated SHR, indicating effectiveness of ganglionic blockade. 7. Unlike the results in genetically hypertensive animals, in normotensive WKY rats pretreated with captopril (30 mg/kg, i.v.; MABP = 81 +/- 4 mmHg; n = 6), or hydralazine (5 mg/kg, i.p.; MABP = 63 +/- 4 mmHg; n = 4) or chlorisondamine (10 mg/kg, s.c.; MABP = 63 +/- 4 mmHg; n = 5), P32/98 did not affect arterial blood pressure. 8. We conclude that, in genetically susceptible animals, inhibition of DPP IV increases arterial blood pressure via Y(1) receptors when elevated blood pressure is reduced with antihypertensive drugs provided that the sympathetic nervous system is functional. The results suggest vigilance because DPP IV inhibitors are used more widely in hypertensive patients treated with antihypertensive drugs.

Histamine in Macaca mulatto monkey cardiac sympathetic nerve system: A morphological and functional assessment

Our previous study demonstrated the co-localization of histamine with norepinephrine (NE) within superior cervical ganglia (SCG), and the release of histamine from sympathetic nerve endings of guinea pig evoked by stimulations. We have now further investigated that whether the histamine can be synthesized, stored and released from the sympathetic nerve systems of Macaca mulatto monkey, and investigated the modulation of the sympathetic endogenous histamine release through histamine H 3 receptor in the monkey cardiac sympathetic nerve system. Double-labeled immunofluorescence technique was applied to investigate co-localization of histamine and NE in SCG of Macaca mulatto monkey. The cardiac sympathetic nerve terminals (synaptosomes) of Macaca mulatto monkey was prepared and depolarized with 50 mmol/L K +. Histamine released from synaptosomes was detected by spectrofluorometer and regulations of histamine release through Ca 2+, Ca 2+-channel blockers, H 3-receptor agonist ( R)-α-methylhistamine and histamine H 3-receptor antagonist, thioperamide were observed. Co-localization of histamine and NE was identified within the same neuron of SCG. Release of histamine was Ca 2+-dependent and inhibited by N-type Ca 2+-channel blocker ω-conotoxin, but not affected by the L-type Ca 2+-channel blocker lacidipine. Compound 48/80, a mast cell releaser, did not affect cardiac synaptosome histamine exocytosis. Cardiac synaptosome histamine release was augmented by the enhanced synthesis of histamine or the inhibition of histamine metabolism. Histamine H 3-receptor activation by ( R)-α- methylhistamine inhibited high K +-evoked histamine release and thioperamide blocked the effects of ( R)-α-methylhistamine. These results firstly showed that histamine co-existed with NE within sympathetic neurons of monkey and the exocytosis of histamine from sympathetic terminals could be regulated by presynaptic histamine H 3 receptors. Sympathetic histamine may act as a neurotransmitter to modulate sympathetic neurotransmission.